With the recent remarkable development of potable electronic equipment, communications equipment and the like, a strong demand for high energy density secondary batteries exists from the standpoints of economy and size and weight reductions. One prior art method for increasing the capacity of secondary batteries is to use oxides as the negative electrode material, for example, oxides of V, Si, B, Zr, Sn or the like or complex oxides thereof (see JP-A 5-174818 and JP-A 6-060867 corresponding to U.S. Pat. No. 5,478,671), metal oxides quenched from the melt (JP-A 10-294112), silicon oxide (Japanese Patent No. 2,997,741 corresponding to U.S. Pat. No. 5,395,711), and Si2N2O and Ge2N2O (JP-A 11-102705 corresponding to U.S. Pat. No. 6,066,414). Conventional methods of imparting conductivity to the negative electrode material include mechanical alloying of SiO with graphite, followed by carbonization (see JP-A 2000-243396 corresponding to U.S. Pat. No. 6,638,662), coating of silicon particles with a carbon layer by chemical vapor deposition (JP-A 2000-215887 corresponding to U.S. Pat. No. 6,383,686), and coating of silicon oxide particles with a carbon layer by chemical vapor deposition (JP-A 2002-42806). None of these patents relates to the method of alleviating a substantial volume change of a silicon negative electrode during charge/discharge cycles which is an outstanding problem characteristic of the silicon negative electrode nor the method of reducing the current collection associated with the volume change. It remains an important task to establish such techniques.
The prior art methods using silicon as such or making its surface conductive for improving the cycle performance of negative electrode material are successful in increasing the charge/discharge capacity and energy density, but are not necessarily satisfactory because of failure to fully meet the characteristics required in the market, particularly the cycle performance of importance in mobile phone and other applications. There is a desire for further improvement in cycle performance.
In particular, Japanese Patent No. 2,997,741 uses silicon oxide as the negative electrode material in a lithium ion secondary cell to provide an electrode with a high capacity. As long as the present inventors have confirmed, there is left a room for further improvement as demonstrated by a still high irreversible capacity on the first charge/discharge cycle and cycle performance below the practical level. With respect to the technique of imparting conductivity to the negative electrode material, JP-A 2000-243396 suffers from the problem that solid-to-solid fusion fails to form a uniform carbon coating, resulting in insufficient conductivity. In the method of JP-A 2000-215887 which can form a uniform carbon coating, the negative electrode material based on silicon undergoes excessive expansion and contraction upon adsorption and desorption of lithium ions, meaning impractical operation, and loses cycle performance. Thus, the charge/discharge quantity must be limited. In JP-A 2002-42606, despite a discernible improvement of cycle performance, due to precipitation of silicon crystallites, insufficient structure of the carbon coating and insufficient fusion of the carbon coating to the substrate, the capacity gradually lowers as charge/discharge cycles are repeated, and suddenly drops after a certain number of charge/discharge cycles. This approach is thus insufficient for use in secondary cells.